Project Summary: Standing and walking are almost always completed in unison with other cognitive tasks such as talking, reading or making decisions. The ability to perform this important type of ?dual tasking? is critical to daily activities and dependent upon one?s capacity to effectively activate appropriate brain networks that include the left dorsolateral prefrontal cortex (dlPFC). Transcranial direct current stimulation (tDCS) is a safe, noninvasive technology that can selectively modulate brain excitability (i.e., the likelihood of activation) by passing low-level currents between electrodes placed upon the scalp. We have demonstrated through a series of studies that a single, 20-minute exposure of tDCS targeting the left dlPFC?administered via two large sponge electrodes?reduces dual task costs to metrics of standing postural control and gait, when tested immediately following stimulation. Still, we and others have also observed relatively high between-subject variability in the effects of this ?traditional? bipolar form of tDCS. We contend that this variability in effectiveness arises in part from relatively diffuse and unspecific current flow when using large sponge electrodes, in combination with individual variability in head and brain anatomy that significantly alters current flow and the generated electric field in the target brain region. In this project, we will apply recent advances in tDCS modeling and administration to 1) model the electric fields generated by traditional tDCS in older adults using their individual structural brain MRIs, and 2) develop personalized tDCS?delivered via an array of eight small gel electrodes?by using optimization algorithms to determine electrode placement and current parameters needed to generate desired electrical field with the brain region of interest. Our Specific Aim is to examine the immediate after-effects of personalized tDCS, traditional tDCS, and sham stimulation on dual task standing and walking in older adults. Our study population will be older men and women without overt disease or illness, yet with poor baseline dual task performance defined as a dual task cost (i.e., reduction) to gait speed of at least 20% induced by simultaneously performing a serial subtraction task when walking. We hypothesize that across participants, the effect of traditional tDCS on dual task standing and walking performance will correlate with a specific component of the electric field generated over the left dlPFC target. We also hypothesize that personalized tDCS will induce A) greater effects on dual task standing and walking performance as compared to traditional tDCS and sham stimulation, and B) these effects will be more consistent across individuals as compared to traditional tDCS. This project will provide important insights into tDCS ?dosage? that will enable us and many other researchers to better understand, control, and optimize this form of noninvasive brain stimulation to individual head and brain anatomy. It is also expected to demonstrate that personalized tDCS, as compared to the traditional approach, significantly improves the size and consistency of observed benefits to dual task standing and walking in vulnerable older adults.